Method for making elemental semiconductor mirror for vehicles

ABSTRACT

A elemental mirror for vehicles having a luminous reflectance of at least about 30% includes a substrate coated with a thin layer of elemental semiconductor having an index of refraction of at least 3 and an optical thickness of at least about 275 angstroms. Preferably, the elemental semiconductor coating is sputter coated silicon or germanium and a light absorbing coating is included therebehind. The mirror is spectrally nonselective with elemental semiconductor optical thicknesses of about 275 to 2400 angstroms on the front substrate surface. Spectrally selective mirrors are provided by adding an interference coating to the elemental semiconductor layer coating, preferably of a dielectric such as silicon dioxide or silicon nitride, on either the front or rear substrate surface, or by using a thicker, single elemental semiconductor layer. Instead of an absorbing coating behind the mirror, additional elemental semiconductor and dielectric thin layers may be included to reduce secondary reflections. The method includes coating the thin elemental semiconductor layer on flat glass and heating to harden the layer and make it more scratch resistant, or heating and bending the glass without destroying the reflective properties of the mirror. The thin interference layer, secondary reflection reducing layers, and/or light absorbing coating may be coated before or after heating and bending.

BACKGROUND OF THE INVENTION

This invention relates to vehicular mirrors and, more particularly, torearview mirrors for vehicles which incorporate a thin layer of anelemental semiconductor to provide luminous reflectance levels adaptedto reduce glare while maintaining visibility.

Vehicular rearview mirrors, especially for the exterior of an automobileor truck, are broadly classified as either spectrally nonselective,i.e., achromatic, metallic or silvery in appearance, or spectrallyselective, i.e., those which use light interference to enhancereflectance in some portion of the visible wavelength spectrum relativeto other portions. For example, a commonly available first surface,chromium coated glass mirror is a spectrally nonselective or metallicappearing mirror. Commercially available blue mirrors which enhancereflection in the blue region of the visible spectrum are exemplary ofspectrally selective mirrors.

It is desirable in both spectrally nonselective and selective vehicularmirrors to reduce glare and provide an antidazzling effect whilemaintaining sufficient luminous reflectance to provide a proper image.Such an image is bright enough that the driver can quickly, accuratelyand easily gather information about the environment even in low lightlevel conditions, but not so bright as to act as a source of glare fromfollowing headlights at night. Luminous reflectance of rearview mirrorsis measured by using a light source which models that from a headlightand by using a detector with a filter which mimics the spectralselectivity of the human eye in its day adapted (photopic) mode.Measurements of luminous reflectance are performed in accordance withSAE (Society of Automotive Engineers) Recommended Practice J964 formeasurement of rearview mirror reflectivity. In the United States,governmental regulations such as Federal Motor Safety Standard 111require a minimum mirror luminous reflectance of at least 35%. InEurope, European Economic Community Council Directive 71/127/EECrequires a similar minimum luminous reflectance of at least 40% forvehicular mirrors. On the other hand, a maximum luminous reflectance of60% to 65% has been found acceptable for glare reducing rearview mirrorsas compared to the luminous reflectance of a full reflectivity mirror ofabout 80% to 90% of the incident light.

In addition, spectrally selective mirrors may be used to furtheroptimize human sight in low light level or night conditions. Asindicated above, luminous mirror reflectance depends both on the type oflight source projecting light on a mirror as well as the type ofdetector which senses the reflected light. The human eye is a detectorwhich adapts to various levels of ambient light by changing itssensitivity to various colors. During the day when light is abundant,human eye sensitivity is highest in the green spectral regions. As lightlevel drops, however, the peak eye sensitivity moves toward the shorter,blue wavelengths. Since headlights have a spectral emission that isstrong in longer yellow wavelengths but weaker in blue, a glare-reducingor antidazzling mirror which optimizes low light vision shouldaccentuate reflectance in the blue regions [400 to 500 nm. wavelengthsor thereabouts] where the eye is most sensitive but reduce reflectancein the yellow regions [above 560 nm. wavelengths or thereabouts] therebyreducing headlight reflectance. Such a mirror is, therefore, spectrallyselective and blue in color.

In the past, both spectrally selective and spectrally nonselectivevehicular mirrors have employed coatings of metal, dielectric materialsor combinations thereof on glass or other substrates. While suchmetal/dielectric layers have functioned adequately, various embodimentshave been expensive to manufacture due to the necessary coating,cutting, bending and heating procedures. Moreover, in many prior knownmirrors, substantial thicknesses of metal or dielectric coatings havebeen required to provide optical thicknesses necessary for properreflectance or spectral selectivity. Increased thicknesses requireadditional material and add expense to production costs.

Also, many vehicle manufacturers specify first surface mirrors on thevehicle exterior in order to reduce ghost or secondary reflections andimages. In such mirrors, the reflective coatings are exposed to theelements and can degrade more quickly than second surface mirrors.

Moreover, the manufacturing processes necessary to make prior knownspectrally selective and nonselective mirrors have often requiredcostly, time consuming procedures which require heating and bending ofglass prior to applying any coating. As is well known, the coating of acurved substrate with a uniform thickness thin layer is more difficult,time consuming and expensive than coating flat glass because of specialfixturing required and the difficulties of cleaning curved surfaces, forexample. Again, such procedures increase production costs.

Further, the production of prior known spectrally selective andnonselective mirrors has often required greatly differing combinationsof layers and materials. The use of one or a few types of layers toproduce spectrally selective as well as nonselective vehicular mirrorswas difficult. Hence, modifying production techniques to incorporate thevarying types of materials and to switch between the differing materialsat different times reduced production efficiency and added to costs.

Thus, the need has remained for a commercially acceptable vehicularmirror which can be economically produced from a material which providesboth spectrally selective and nonselective mirrors, allows use on bothfirst and second surface mirrors, and provides luminous reflectancesmeeting worldwide minimum safety standards while maintaining desiredglare reduction.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a solution to the aboveneeds with a glare-reducing mirror for vehicles having a luminousreflectance of between about 30% and 65% to achieve consumer andregulatory acceptability with either spectrally selective ornonselective mirrors. The invention achieves this result through the useof a thin layer of an elemental semiconductor which may be used alone invarying thicknesses or combined with interference coatings preferably ofa dielectric material, thereby producing both spectrally selective andnonselective mirrors.

Use of elemental semiconductors provides high refractive indexesallowing suitable optical thicknesses with thinner material layers and,therefore, increased production economies. Optical thickness isimportant in the present invention because optical thickness controlsthe interference of light which produces the desired spectral responsein the mirrors herein. In addition, manufacturing flexibility isprovided since the semiconductor layers can be provided in varyingthicknesses for use on either first or second surface mirrors, forspectral selectivity or nonselectivity, for combination withinterference coatings to produce spectrally selective mirrors as well,or for combination with dielectric layers in additional coatings whichreduce secondary reflections from the mirror. In addition, thesemiconductor and other layers are easily coated on flat glass andsubsequently heated to improve their scratch resistance and hardness orheated and bent to produce a curved mirror, all without reducingreflective qualities.

Conventional mirror coatings of metal like silver, aluminum, chromium,titanium, stainless steel or the like do not withstand elevatedtemperatures in manufacturing or use (e.g., temperatures encounteredduring bending operations) whereas coatings in the present invention do.Nor do the conventional coatings withstand such conditions even afterbeing deposited in combination with other thin films to form multilayerstacks. Therefore, the present invention utilizes elementalsemiconductors such as silicon or germanium as thin film materials thatretain beneficial cosmetic, reflective and mechanical (i.e., does notcrack or flake off the substrate) properties even after heating totemperatures sufficient to permit bending of the glass substrate.

Some metals, like silver and aluminum, are used only in second-surfacevehicular mirror constructions because, without proper protection, theydeteriorate rapidly in out-of-doors environments. Such coatings are,therefore, overcoated with paint, tape or the like to protect the metalfrom exposure to the environment. Scratch resistance is thereby providedby the substrate glass. First surface mirrors use metals like chromium,titanium, stainless steel or the like because such coatings adequatelyresist scratches and wear and because they do not deteriorate as rapidlyas silver and aluminum when exposed to the environment typicallyencountered outside automobiles or trucks. The present invention issurprisingly and beneficially more resistant to scratching and wear thanthe conventional mirror coatings and so provides consumer appreciablebenefit in first surface embodiments over conventional first surfacemirrors in that the coatings of this invention are more rugged whenexposed to abrasives such as are found in car washes and the like.

In one form, the invention is a glare-reducing mirror for vehiclescomprising a substrate having front and rear surfaces and a thin layerof an elemental semiconductor on one surface of the substrate. Theelemental semiconductor coating has an index of refraction of at least 3and an optical thickness of at least about 275 angstroms. The thinelemental semiconductor layer provides the mirror with a luminous lightreflectance of at least about 30% of the light incident thereon from thedirection of the front surface of the substrate.

Preferably, the elemental semiconductor is silicon or germanium. Whensilicon is used in an optical thickness of between about 345 and 2400angstroms, the elemental semiconductor layer provides a spectrallynonselective mirror with luminous light reflectance between about 30%and 65%. When germanium is used in an optical thickness of between about275 and 2400 angstroms, a similar spectrally nonselective mirror isproduced. However, when the thickness of the elemental semiconductorlayer is increased to between about 2400 and 10,000 angstroms, themirror becomes discernibly spectrally selective.

In another aspect of the invention, a spectrally selective,glare-reducing mirror is provided including a substrate having front andrear surfaces and a single, thin layer of an elemental semiconductor anda single, thin, transparent, interference coating on the elementalsemiconductor thin layer. Again, the elemental semiconductor thin layerhas an index of refraction of at least 3 and an optical thickness of atleast about 275 angstroms. The interference coating has an index ofrefraction within the range of between about 1.3 and 2.7, an opticalthickness of at least about 500 angstroms, and is positioned closest toa source of incident light to be reflected by the mirror. Such mirrorhas a luminous reflectance of at least about 30% of the light incidentthereon from the direction of the front substrate surface.

In the above mirror, thinner interference coatings than about 500angstroms do not give appreciable spectral selectivity. Opticalthicknesses between about 1600 and 2800 angstroms are preferred becausethey result in good spectral selectivity. Above 2800 angstroms, spectralselectivity is maintained with a more complex pattern of maxima andminima appearing in the spectral reflectance.

In a preferred form, the elemental semiconductor thin layer may beapplied to the front substrate surface with the interference coatingbeing applied over the semiconductor thin layer. Alternately, when thesubstrate is transparent, the interference coating may be applied to thesubstrate rear surface with the semiconductor thin layer being appliedover the interference coating to the rear of the substrate. In eithercase, the elemental semiconductor thin layer may be selected from thegroup including silicon and germanium while the interference coating ispreferably selected from the group including silicon dioxide and siliconnitride. With silicon in an optical thickness between about 345 and 1200angstroms, and an interference coating of silicon dioxide or siliconnitride having an optical thickness within the range of between about1600 and 2800 angstroms, the mirror provides a blue colored reflectance.

In all cases, a light absorbing layer may be applied to the rear of thesubstrate or to the last coating on the coated mirror to preventsecondary reflections that would detract from the performance of theoverall mirror. Such layer may be chosen to provide anti-scatterprotection for pieces or fragments of the substrate in the event thatthe substrate is broken. Alternately, to reduce secondary reflections,additional thin film layers of an elemental semiconductor and adielectric may be coated on the front or rear surfaces of the othercoating layers for opacification or near-opacification of the mirror.

In yet another aspect of the invention, a method for manufacturing aglare-reducing mirror for vehicles includes providing a sheet of flatglass having front and rear surfaces, coating one surface of the sheetwith a thin layer of an elemental semiconductor having an index ofrefraction of at least 3 and a desired optical thickness of at leastabout 275 angstroms, heating the coated glass to a temperaturesufficient to allow bending of the coated glass and bending the heated,coated glass to a desired curvature. Alternately, a thin layer ofdielectric material may be coated on the elemental semiconductor layerprior to heating and bending thereby producing a spectrally selectivemirror.

Alternatively, the rear surface of the substrate may be coated with athin layer of dielectric material followed by the thin elementalsemiconductor layer prior to heating and bending. In each of thesecases, the subsequent heating and bending does not significantly degradethe reflective characteristics of the coated mirror.

In other aspects, the method for manufacturing a glare-reducing mirrorfor vehicles includes providing a sheet of flat glass, coating onesurface of the sheet with an elemental semiconductor layer as describedabove, and heating the coated glass for a limited period of time tocause the semiconductor coating to be environmentally resilient, hardand scratch resistant. Alternately, the flat glass may be heated priorto application of the semiconductor coating to also produce the hard,scratch resistant coating result.

In addition, the present method includes coating a flat glass sheet withan elemental semiconductor coating, or an interference coating such assilicon nitride, or coatings of both, on either the front or rearsurfaces of the glass, followed by heating and bending the coated glass.After cooling, if only one coating has been applied, the other materialcan subsequently be coated in a thin layer to produce a glare-reducingmirror.

Accordingly, the present invention provides both spectrally selectiveand nonselective mirrors using a thin layer of an elementalsemiconductor which are more economical to produce while providingcommercially acceptable luminous reflectances which also meet minimumgovernmental standards. The thin elemental semiconductor layer providesenvironmental resilience and flexibility for use either alone or withdielectric interference coatings resulting in either first or secondsurface and spectrally selective or nonselective mirrors as desired.Moreover, such coatings may be combined with dielectric layers and otherelemental semiconductor layers to produce spectrally selective mirrors,or to produce either spectrally selective or nonselective mirrors thatare opacified using thin films. All these stacks of coatings can beheated to improve scratch resistance, or coated on a heated substrate toachieve the same effect, or heated and bent to form curved mirrors.

These and other objects, advantages, purposes and features of theinvention will become more apparent from a study of the followingdescription taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of one form of a spectrally nonselectiveglare-reducing vehicular mirror of the present invention;

FIG. 2 is a sectional view of the mirror of FIG. 1 taken along planeII--II;

FIG. 3 is a graph of the spectral response for a mirror of the typeshown in FIGS. 1 and 2 including a silicon reflective layer of anoptical thickness of about 1520 angstroms;

FIG. 4 is a graph of the optical interference effects of varying opticalthickness on luminous reflectance for elemental silicon thin films onglass, computed using the dispersion of the refractive index.

FIG. 5 is a sectional view of a second embodiment of a spectrallynonselective vehicular mirror of the present invention;

FIG. 6 is a sectional view of a third embodiment of a spectrallynonselective, convex vehicular mirror of the present invention;

FIG. 7 is a graph of the spectral response for the spectrallynonselective mirror of FIG. 6 including an elemental silicon layer of anoptical thickness of about 1600 angstroms;

FIG. 7A is a sectional view of a fourth embodiment of a spectrallynonselective, convex vehicular mirror of the present invention;

FIG. 8 is a sectional view of a fifth embodiment of a second surfacespectrally nonselective mirror of the present invention;

FIG. 9 is a graph of the spectral response for the mirror of FIG. 8including an elemental silicon layer of an optical thickness of about800 angstroms, followed by a layer of silicon dioxide of an opticalthickness of about 1500 angstroms, followed by a second layer ofelemental silicon of an optical thickness of about 7200 angstroms;

FIG. 9A is a sectional view of a sixth embodiment of the inventioncomprising an achromatic, spectrally nonselective, first surface, convexvehicular mirror;

FIG. 9B is a graph of the spectral response for the achromatic mirror ofFIG. 9A including a coating of elemental silicon on glass of about 6950angstroms, a thin film of silicon dioxide of about 1050 angstroms, and asecond thin film of elemental silicon of about 1600 angstroms as theoutermost layer;

FIG. 10 is a sectional view of a seventh embodiment of a spectrallyselective vehicular mirror of the present invention;

FIG. 11 is a sectional view of a eighth embodiment of a spectrallyselective vehicular mirror of the present invention;

FIG. 12 is a graph of the spectral response for a spectrally selectivemirror similar to that shown in FIG. 10 including a multilayer coatingof an optical thickness of about 2160 angstroms silicon dioxide sputtercoated atop a layer of silicon of an optical thickness of about 880angstroms on glass;

FIG. 13 is a graph of the spectral response for a prior known,commercially available, spectrally selective mirror sold by DonnellyCorporation of Holland, Mich., U.S.A.;

FIG. 14 is a sectional view of an ninth embodiment of a spectrallyselective vehicular mirror of the present invention;

FIG. 15 is a graph of the spectral response for the mirror of FIG. 14including a coating of silicon of about 1200 angstroms optical thicknesson glass followed by a layer of silicon nitride having an opticalthickness of about 2000 angstroms;

FIG. 16 is a sectional view of a tenth embodiment of a second surfacespectrally selective vehicular mirror of the present invention;

FIG. 17 is a graph of the spectral response for the mirror of FIG. 16including a coating of silicon nitride having an optical thickness ofabout 2200 angstroms on glass followed by a layer of silicon having anoptical thickness of about 1200 angstroms and a layer of silicon nitridehaving an optical thickness of about 2400 angstroms;

FIG. 18 is a sectional view of a eleventh embodiment of a spectrallyselective vehicular mirror of the present invention;

FIG. 19 is a graph of the spectral response of the mirror of FIG. 18including a single layer of silicon having an optical thickness of about4800 angstroms on glass;

FIG. 20 is a front view of a combined vehicular mirror including a lowerpanel comprising the mirror of FIG. 18 and a second, upper panel havinga higher luminous reflectance than that of the lower panel;

FIG. 21 is a sectional view of an twelfth embodiment of a first surfacespectrally selective vehicular mirror of the present invention; and

FIG. 22 is a graph of the spectral response of the mirror of FIG. 21including a layer of elemental silicon having an optical thickness ofabout 6800 angstroms coated on glass, a layer of silicon dioxide infront of the silicon layer and having an optical thickness of about 1050angstroms, a second layer of elemental silicon in front of the silicondioxide and having an optical thickness of about 800 angstroms, and asecond layer of silicon dioxide in front of the second silicon layer andhaving an optical thickness of about 2250 angstroms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in greater detail, FIGS. 1 and 2illustrate a first embodiment 10 of the glare-reducing mirror of thepresent invention adapted for use as an exterior, rearview vehicularmirror for automobiles, trucks or the like. Mirror 10 includes a glasssubstrate 12 formed from optically clear, transparent, float glass or,alternately, tinted or light absorbing glass cut to the shape of anexterior rearview mirror, in this case, one adapted for use on thedriver's/left side of a vehicle. In addition, optically clear or tintedor light absorbing plastic substrates formed from polycarbonate,acrylic, polystyrene, allyl diglycol carbonate, styrene acrylonitrile,polysulfone and the like may be used in this invention. Preferably,substrate 12 has flat or planar front (or first) and rear (or second)surfaces 14, 16 which are generally parallel to one another and aperipheral edge surface 18, although the invention can be used on glassor other substrates having nonparallel surfaces forming prismaticelements typically used in day/night rearview mirror applications, or onglass surfaces that are nonparallel for other reasons. Alternately,substrate 12 may be bent or curved to a desired radius as describedbelow.

As is shown best in FIG. 2, a thin layer 20 of a partially transparent,high refractive index, elemental semiconductor preferably selected fromsilicon and germanium is coated to a desired thickness on the front orfirst surface 14 of glass substrate 12. The thickness of layer 20 hasbeen exaggerated for clarity as are the thicknesses of the othercoatings and substrates for all the embodiments herein. The viewingdirection for the various mirrors is indicated by the arrow in thefigure for each embodiment. On the rear surface 16, a protective coating22 of light absorbing material is applied over the entire surface toabsorb and prevent reflection of light transmitted through thesemiconductor layer 20 and substrate 12 and prevent secondaryreflections which would detract from the reflective quality of mirror10.

The elemental semiconductor layer 20 of mirror 10 is preferably a vacuumsputtered coating of silicon having an optical thickness within therange of between about 345 and 2400 angstroms. This corresponds to acoating thickness of between about 85 and 600 angstroms at an index ofrefraction (n) of 4. Specifically, an optical thickness of between about400 and 2400 angstroms of silicon elemental semiconductor may be coatedonto flat or curved substrate 12. This corresponds to a coatingthickness of between about 100 and 600 angstroms at n=4. The preferredrange of optical thickness of elemental silicon for mirror 10 is betweenabout 800 and 1800 angstroms. An optical thickness of about 960angstroms is most preferred corresponding to a coating thickness ofabout 240 angstroms at n=4. After washing the substrate byconventionally known, standard glass substrate washing methods, varioustypes of vacuum deposition techniques may be used. Preferably, directcurrent (DC) magnetron vacuum sputtering from a silicon elementalsemiconductor target preferably doped with another element or combinedwith a metal to improve electrical conductivity is used. For example, asilicon sputtering target doped with 200 parts per million (ppm)phosphorous may be used, as can a sputtering target of silicon dopedwith aluminum. The level of 200 ppm phosphorous is near the soluabilitylimit in silicon. Electrical conductivity is generally lower for smallerconcentrations of phosphorous. Thus, 200 ppm is preferred as it providesgood electrical conductivity for the silicon sputtering target, which isespecially useful when DC sputtering. If the aluminum concentration inthe sputtering target is kept below about 15%, it has been found thatsubsequent heating and bending processes do not cause any significantloss of reflectance from the mirror and, in fact, increase the resultingreflectance somewhat. Lower target concentrations of aluminum alsoresult in acceptable performance for silicon elemental semiconductorfilms. Aluminum concentrations above about 15% in sputtering targets ofelemental silicon are acceptable for some applications in the presentinvention where exposure to temperatures above about 200° C. is notanticipated. However, the resulting reflectance loss with such higheraluminum concentrations makes such concentrations less preferred formost embodiments of this invention. Also, it has been found that filmssputtered from phosphorous doped silicon targets are less resistant tosalt spray damage during environmental testing. Therefore, elementalsilicon coatings from phosphorous doped targets are less preferred forfirst surface mirror applications but useable for second surfacevehicular mirrors as described below where the substrate providesprotection for the thin films from the environment. Elemental siliconcoatings from sputtering targets doped with aluminum are lesssusceptible to such salt spray damage.

Moreover, other dopants such as gallium, boron, arsenic and the like canbe used as well. Suitable results have also been obtained using plasmasprayed sputtering targets using either a 200 ppm phosphorous-dopedsilicon composition or a composition of 6% aluminum and 94% silicon ofthe type manufactured by Plasmaterials, Inc. of San Ramon, Calif. Plasmaspraying is a particularly good process for making sputter targets whichhave geometrics nonamenable to conventional sputter target applicationand can be more economical in that sputtered regions in a used targetcan be resprayed and filled in instead of replacing the whole target.Suitable results are also obtained using targets formed from aphosphorous doped, crystalline silicon manufactured by AdvanceInternational Materials, Inc. of Suffern, N.Y. using a more conventionalsputtering target manufacturing process.

It is desirable that any mirror made in accordance with the presentinvention have achromatic spectral characteristics if it is to be usedas a substitute for currently available, commercially acceptable,chromium coated, spectrally nonselective exterior vehicular rearviewmirrors. Achromatism is demonstrated by reflectance which remainsrelatively constant across most of the visible wavelength region of thelight spectrum, i.e., between about 400 and 750 nanometers (nm).

With reference to FIG. 3, the spectral response for a specific rearviewmirror like that in FIGS. 1 and 2 made by coating elemental siliconsemiconductor to an optical thickness of about 1720 angstromscorresponding to a coating thickness of 380 angstroms at n=4 onto oneglass surface is shown. The coating was made with a vacuum sputteringtarget of 6% aluminum/94% silicon composition sputtered at a pressure of3 mTorr in a chamber containing neon gas by DC planar magnetronsputtering. Thereafter, the coated glass was bent using a conventionalheating and bending process to a radius of curvature of about 40 inches.Bending is conventionally done by heating the glass to at least 450° C.for a period of time sufficient to permit conforming the glass to a moldas is known to those skilled in the art of glass bending. Typically,soda-lime glass of a thickness of about 0.093 inches which is preferablyused for the embodiments of the present invention described herein isheated between several minutes and an hour at a temperature of at least450° C. in an oven or furnace to permit bending. As shown in FIG. 3, thereflectance differs by no more than about 11% across the entire visiblewavelength range as measured. In appearance, the mirror is silvery incolor, i.e., spectrally nonselective and achromatic. Its luminousreflectance is 63.2% measured from the front or first surface side, andis 57.4% as measured from the rear or second surface side. The luminoustransmittance is 24% measured from either side. Of course, thicker orthinner coatings can be made to have lower reflectance which alsofulfill the object of the present invention.

The elemental semiconductor mirrors of the present invention are morescratch resistant than conventional first surface mirrors, whether ornot the mirror of the present invention is heated. This is shown byrubbing each with fine steel wool. A first surface achromatic mirror ofthe present invention that was heated and bent to form a convex mirrorwas rubbed 4000 strokes with fine steel wool and showed no visibledamage on close visual inspection. A chromium coated glass first surfacemirror subjected to the same test but for only 1000 strokes showedsignificant damage that was easily visible. This consumer appreciablebenefit of enhanced scratch and wear resistance of the present inventionalso permits using first surface constructions where second surfaceconstructions may have been selected. A further benefit is then realizedin that using first surface rearview mirrors avoids a common undesirabletrait of second surface mirrors, i.e. double imaging (reflections fromboth the front glass surface and the rear mirror coated surfacesimultaneously). Further, first surface use of the elementalsemiconductor of this invention, besides enabling enhanced scratch andwear resistance vis-a-vis metal films, as conventionally used, alsobenefit from consumer desirable wetting characteristics such that raindrops and the like tend to bead on the elemental semiconductor coatedsurfaces. On a second surface mirror, raindrops tend to wet and spreadout over the glass front surface due to surface energy effects and thelike. This can lead to blurred image with increased glare, reduced rearvision and increased eye strain therefrom. By contrast, the beading ofwater droplets upon an elemental semiconductor coated front surfacereduces these effects and the water beads tend to run off the coatedfront surface of the mirror surface by wind action and the like. Thus,these elemental semiconductors are useful as first surface, reducedwetting mirror reflectors of enhanced scratch and wear resistancecompared to conventionally known first surface reflectors such aschromium and the like.

Adhesion of a coating to the substrate or of a coating to an alreadydeposited coating can be enhanced if desired by suitable changes to thevacuum parameters. For example, during deposition, the oxygen partialpressure in the coating chamber can be temporarily increased duringformation of the first several monolayers adjacent the substratesurface. This assists in improving adhesion. Similarly, the same can bedone during formation of other layers as well with similar results.

In mirror 10, and that of FIG. 3, the elemental silicon layer has anindex of refraction (n) equal to or greater than 3. In fact, some reportrefractive indexes for elemental silicon exceeding 4 in the visiblespectrum. The use of silicon with such an index of refraction providesan optical thickness, defined as the product of the layer thickness andthe refractive index, which reduces the cost of the present inventivemirror. For example, a thin film having a coating thickness of 1000angstroms and a refractive index of 3 has an optical thickness of 3times 1000 angstroms or 3000 angstroms. Thus, with a high index ofrefraction as with elemental silicon semiconductor, thinner physicallayer thicknesses can be used making the mirror less costly to produce.

Optical thickness is an important measurement employed to characterizeinterference effects which determine the spectral response, i.e., mirrorreflectance. Since the present invention uses optical interference,optical thickness is an important consideration in the present mirrors.The optical thickness of a coating can vary across the visiblewavelength region of the light spectrum because the refractive index maydiffer from wavelength to wavelength of light. This phenomenon is knownas dispersion. For amorphous elemental silicon in the visible wavelengthregion, the refractive index varies from n=4.38 at a wavelength of 413nm to n=3.93 at a wavelength of 775 nm. Therefore, a typical refractiveindex of n=4 may be assumed to be representative of the refractive indexof the deposited elemental silicon in the present invention. The opticalthickness based on such a representative index of refraction will befour times the physical thickness. The exact optical thickness dependson the wavelength of light employed for the measurement via therefractive index at that wavelength. Moreover, the refractive indexeslisted above are for films deposited under certain conditions. Since therefractive index depends on the deposition conditions, it is preferredto set some limits on the refractive index rather than taking therefractive index to be a specific value for all wavelengths. In general,the refractive index of elemental semiconductors is above n=3.0 over thevisible wavelength region of the optical spectrum. The present inventionutilizes this substantially high refractive index of elemental siliconor germanium thin film without requiring any precise or accuratelydetermined specific values of such refractive index.

FIG. 4 illustrates the effects of optical thickness on the luminousreflectance observed for elemental silicon films on glass, computedusing the dispersion of refractive index from which the above refractiveindexes were obtained. The bottom axis gives the physical coatingthickness and the top axis gives the optical thickness when one assumesthat the refractive index is n=4 over the region where luminousreflectance is being measured. The oscillations in the curve from 0 toabout 2500 angstroms coating thickness, or between 0 and 10,000angstroms optical thickness, result from optical interference. The highreflectances above 60% occur for coating thicknesses of near 400angstroms. The interference pattern damps out with greater thickness.This latter effect is due to a dispersive phenomenon called absorption.Whereas dielectric materials typically remain substantially transparentover many thousands of angstroms thickness, and metals typically becomeopaque or near opaque within about 2000 angstroms optical thickness,elemental silicon goes from highly transmissive at near zero thicknessto nearly opaque at about 10,000 angstroms optical thickness. At thispoint, reflectance becomes constant and does not vary with heaviercoating thicknesses. This condition is referred to as near opacity inlater embodiments of the present invention.

FIG. 4 also shows that optically opaque or nearly opaque coatings ofelemental silicon will have about 40% reflectance, which meets therequirements for luminous reflectance of vehicular rearview mirrors, butwill have a grayish cast in reflectance. Thinner layers used in manyembodiments of the present invention have a distinctively silveryappearance. When thicker elemental silicon layers are used foropacification or near opacification in the present invention, thesilvery color is maintained by using silicon dioxide or other thin filmdielectric materials in conjunction with the thick silicon layer behindthe mirror coatings.

Also, applying elemental silicon to coating thicknesses beyond thosewhere oscillations cease as shown in FIG. 4 is not a particular benefitin the present invention. Indeed, the use of thicker than requiredelemental silicon coatings may potentially increase the likelihood ofthe film cracking, crazing or otherwise being damaged during heating orbending operations as described hereinafter.

Alternately, the mirror embodiment of FIGS. 1-3 can be produced with anelemental germanium semiconductor layer. Elemental germanium thin filmsrequire different thicknesses than those used for elemental siliconlayers for equivalent performance because the index of refraction (n) ofgermanium is higher than that of silicon, i.e., about 5 and ranging fromvalues of about 4.5 to 5.5 in various visible wavelength regions. Avalue of n=5 is used for illustrative purposes herein and to provide asimple conversion between layer thickness and optical thickness. Forexample, germanium layers would preferably require layer thicknesseswithin the range of 160 to 500 angstroms to produce an optical thicknessof between about 800 and 2500 angstroms which is generally equivalent tothat of a silicon layer thickness within the range of about 200 to 600angstroms producing an optical thickness of between about 800 and 2400angstroms.

Layers of silicon or germanium differ in their preferred thicknesses formeeting the objects of the present invention as their dispersions ofrefractive index differ. Achieving a luminous reflectance of about 30%requires an optical thickness of about 345 angstroms of silicon whereas275 angstroms is sufficient for germanium. Likewise, a luminousreflectance of about 35% requires an optical thickness of about 400angstroms of silicon whereas 325 angstroms is sufficient for germanium.Similarly, differences in optical thickness between silicon andgermanium elemental semiconductor layers are required when other levelsof reflectance are desirable. In particular, for germanium elementalsemiconductor layers, the corresponding preferred range of opticalthickness is from about 275 angstroms to about 2500 angstroms.

Light absorbing coating 22 on the rear surface 16 of mirror 10 may beany dark colored, preferably black, or other light absorbing paint orlacquer applied by spraying, roller coating or curtain coating. Lightabsorbing coating 22 will have a thickness dependent on the type ofcoating selected. This thickness will be sufficient to provide anoverall luminous transmittance of about 4% or less, and will typicallybe in the range of 10 microns to 1 mm. For example, a suitableprotective paint is METAL SAVER™ black epoxy coating spray paint #7886,Rust Oleum Corp., Vernon Hills, Ill. When applied to the rear surface ofthe mirror substrate 12, layer 22 absorbs most or all of the lighttransmitted through the semiconductor coating 20. Alternately, pigmentedadhesive, adhesive tape, UV curable pigmented adhesive having a dark,black or other color followed by a black colored or other highlyabsorbing tape or other backing such as ceramic may be applied to therear surface of the mirror to absorb light in the same fashion.Application of a dark colored or black opaque tape or some black or darkcolored hot melt plastic or a dark colored, UV, thermally orcatalytically cured epoxy material to the rear surface of the inventionto a thickness sufficient to keep luminous transmittance of the mirrorto less than about 4% but up to a few millimeters total thickness canprovide not only the desired light absorption, but also scatter proofingsafety. Tapes, plastic coatings or other adhesive systems hold pieces ofthe glass mirror substrate together in the event of breakage due toimpact or other forces. Such scatter prevention prevents injury tovehicle passengers by preventing the scattering of fragments or shardsof glass from the mirror.

It is also possible to incorporate certain dark or black paints whichsurvive high temperatures and yet still function to absorb lighttransmitted through the reflective elemental semiconductor coating 20.Examples of such paints, which may be applied to the flat glasssubstrate 12 and subsequently heated and bent along with thesemiconductor coating 20 are HIGH TEMP GUARD™ black paint manufacturedby Advance Materials of Peachtree City, Ga. and DENPLEX™ #21202 blackhigh temperature paint manufactured by U.S. Packaging Corporation ofWheeling, Ill.

As a further alternative, a slightly modified embodiment 25 of thespectrally nonselective, elemental semiconductor coated, glare-reducingmirror 10 of FIGS. 1-3 may be used as shown in FIG. 5. Mirror 25, wherelike number numerals indicate like parts to those in mirror 10, includesa substrate 12, elemental silicon or germanium semiconductor coating 20adjacent the front surface 14, and a light absorbing or anti-scattercoating 22 applied as described above to rear surface 16 of thesubstrate. In this version, however, a dark or black ceramic thin film27 is applied first to the front surface 14 of substrate 12 over whichthe elemental silicon semiconductor layer 20 is thereafter applied inthe manner described above. Alternately, ceramic thin film layer 27 maybe applied to the rear surface 16 of substrate 12 in place of layer 22to perform the same light absorbing function. Preferably, layer 27 is atitanium/aluminum/oxi-nitride composite layer which is opaque, highlylight absorbing, and black at preferred thicknesses of a few tenths of amicron. It will also be apparent that anti-scatter layer 22 may becoated over ceramic layer 27 when applied to rear surface 16 to provideanti-scatter protection not provided by the ceramic layer. In such case,the optical properties of layer 22 are not important.

Alternately, when elemental silicon semiconductor layer 20 is applied tofirst surface 14 of substrate 12, light absorbing layer 22 may beomitted if substrate 12 is formed from an opaque ceramic or lightabsorbing glass material which itself will absorb light and reducesecondary reflections thereby obviating the need for any additionallight absorbing layer. For example, a sheet of black ceramic tile may beused as substrate 12 to obtain desired performance characteristics forthe mirror shown in FIGS. 1-4.

A second embodiment 30 of the present invention is provided with acoating on the rear or second surface 16' of curved, soda-lime glasssubstrate 12' is shown in FIG. 6. As illustrated, mirror 30 is bent to aconvex form such that the outer glass viewing surface 14' is convexwhile the rear surface 16' is concave. A layer 32 of elemental siliconis applied to an optical thickness between about 480 and 2400 angstroms,and preferably between about 550 and 1600 angstroms for a luminousreflectance of between 35% and 65%, corresponding to a coating thicknessof about between 137 and 400 angstroms at n=4, by DC sputtering in neongas on rear surface 16'. A dark colored backing layer 22 of the typedescribed above is then applied over the silicon coating 32. Asdescribed below, especially in connection with mirror 80 in FIG. 16,backing coating 22 may be extended around silicon layer 32 forprotection against environmental effects although layer 32 issufficiently durable even without such protection. The silicon wassputtered from a 94% silicon/6% aluminum composition sputtering target.

FIG. 7 illustrates the spectral reflectance in the visible wavelengthregion for mirror 30 with an elemental silicon layer 32 of an opticalthickness of about 1600 angstroms and coating 22 as described above. Theluminous reflectance after heating and bending, but before applying darkbacking coating 22, is 61.5% as measured from the concave side of themirror, and 55.9% as measured from the convex or viewing side. Theluminous transmission through the glass and layer 32 is 24.6%. Aftercoating the rear surface with Rust Oleum METAL SAVER™ black epoxycoating spray paint #7886, the luminous reflectance as measured from theconvex side is 47.5%. As shown in FIG. 7, the reflectance varies by nomore than about 16% over the visible spectrum making mirror 30essentially achromatic or spectrally nonselective.

Mirror 30 differs from mirrors 10 and 25 in that the initial reflectingsurface is the glass/silicon interface whereas, in mirrors 10 and 25, itis the air/silicon interface that is first encountered. Less light isreflected in mirror 30 prior to encountering the absorbing effect of thesilicon layer 32 itself. The higher reflectance from the firstencountered surface in mirrors 10 and 25 compared with the firstencountered surface in mirror 30 is due to a larger difference in therefractive indexes at the air/silicon interface than at theglass/silicon interface. Also, dark colored backing 22, if used, impactsthe optical response of mirror 30 since it directly contacts one surfaceof the silicon elemental semiconductor layer 32. The optical constantsof the paint/lacquer/resinous plastic, epoxy cured plastisol or likebacking have some effect on the optical response, i.e., on reflectanceas a function of wavelength, and therefore also on the luminousreflectance of the mirror. Yet, it is possible to achieve luminousreflectances between 55% and 60% by proper selection of the thickness oflayer 32 of the silicon elemental semiconductor and by care andselection of the paint or other dark backing layer 22.

It is also possible to include a low refractive index dielectricmaterial such as silicon dioxide or silicon nitride coated over siliconlayer 32 followed by an opaque thin film or dark backing such as paint,lacquer, epoxy cured plastisol, resinous plastic or the like in order toprovide the desired level of reflectance and the desired spectralnonselectivity or spectral selectivity. For example, a second surfaceachromatic mirror 35 of this construction was made as shown in FIG. 7Aby coating a thin layer 36 of elemental silicon semiconductor onto aglass substrate 12' followed by coating a thin layer 38 of silicondioxide over the silicon, then bending the coated glass, then paintingthe coated side of the construction as shown at 22 in the mannerdescribed above. The elemental silicon semiconductor layer 36 was coatedto an optical thickness of about 750 angstroms, and this was followed bycoating a thin layer 38 of silicon dioxide to an optical thickness ofabout 1600 angstroms. The coated glass substrate was then bent so thatthe thin layers were on the concave surface of the glass substrate.Finally, a black epoxy spray paint coating 22 of Rust Oleum #7886 METALSAVER was applied to the rear of the silicon dioxide layer. This designyielded a luminous reflectance of about 52%, viewed second surfacethrough the glass substrate. Moreover, the mirror was still silvery orachromatic in appearance.

With reference to FIGS. 8 and 9, an alternate form 40 of the mirrorinvention is shown wherein secondary reflections are reduced using thinfilms. Mirror 40 includes highly absorbing thin films which are adjustedand provided in appropriate thicknesses in place of dark colored backinglayers such as that described above at 22 whereby the entire coatedsubstrate essentially is opaque, or near opaque, or low lighttransmitting in nature, i.e., less than 4% luminous transmission. Thisalternate form can be included in both first and second surface mirrorsof both the achromatic or nonspectrally selective and spectrallyselective or blue types of mirrors. If elemental silicon is used as thehighly absorbing thin film, then the coating may be heated and bentwithout sacrificing reflective or other optical qualities as describedbelow. This differs from the use of metals that either will not survivebending or require the use of sacrificial layers to maintain theirintegrity during high temperature bending processes, or merely tosurvive heating in an oven. Using elemental or near elemental siliconthus provides significant advantages over use of metals.

The use of thin films for opacifying mirrors of the present invention asdescribed above can be preferred in order to avoid secondary operationsof applying absorbing coatings or tapes to the mirror rear surface. Forexample, painting can overspray or wick onto the mirror front surface,requiring another face scrubbing or cleaning operation to remove theundesirable overspray. Avoiding overspray or wicking requires extrafixturing and/or care. Also, painted parts are more difficult to cutinto shapes. We find locating the cutting wheel scribe line moredifficult on painted mirrors than on mirrors opacified by the use ofthin film means, resulting in yield loss from errors in breaking the cutshape out of the larger glass lite. Therefore, there are certainadvantages that come from opacification or near opacification by the useof thin film means. Even more advantage is gained because the entireopacified or near opacified mirror of the present invention is bendable,thus permitting single shape bending after cutting the shape from a flatlite of glass. A further advantage of opacification or nearopacification by use of thin film means is that the entire commerciallyuseable mirror comprising the reflective means and the opacification ornear opacification means can be fabricated in an integrated process,such as a thin film deposition chamber, without requiring subsequentprocessing.

As shown in FIG. 8, a specific achromatic, nonspectrally selectivemirror design 40 is illustrated including a two layer combination ofsilicon dioxide or silicon nitride and elemental silicon substituted forthe dark colored backing layer. In this case, the first elementalsilicon layer on glass is adjusted to produce an achromatic secondsurface mirror. As shown in FIG. 8, a layer of elemental silicon 42having an optical thickness of about 800 angstroms, corresponding to acoating thickness of about 200 angstroms at n=4, is sputtered asdescribed above onto rear surface 16 of a flat, soda-lime glasssubstrate 12. Thereafter, a silicon dioxide layer 44 having an opticalthickness of about 1500 angstroms, corresponding to a coating thicknessof about 1000 angstroms at n=1.5, is applied by RF sputtering using asilicon dioxide target in argon sputtering gas at a pressure of 3 mTorras described above. Subsequently, a thicker layer 45 of elementalsilicon is deposited in the same manner as the first elemental siliconlayer 42 but having an optical thickness of about 7200 angstromscorresponding to a coating thickness of about 1800 angstroms at n=4. Inthis form, thicker layers 45 of elemental silicon are acceptable. Theselection of layer thicknesses to provide opacification is made suchthat the overall luminous transmittance is below about 10% andpreferably below about 4% for any mirror of the present invention. Forexample, elemental silicon layers 45 of optical thickness greater thanabout 4000 angstroms or thereabouts achieve overall luminous lighttransmittance of about 10% or less. Use of elemental silicon layers 45of optical thickness greater than about 6800 angstroms will achieveoverall luminous light transmittance of about 4% or less. Whenmanufactured with layer 45 being silicon of optical thickness 7200angstroms, mirror 40 has a luminous reflectance of 61% and a luminoustransmittance of 0.1%. When heated to over 500° C. in less than a minuteby placing into a furnace, the resulting heated coating maintains areflectance of at least 61%, is achromatic or spectrally nonselective,and has a luminous transmittance of only 0.8%. This result isillustrated in FIG. 9 wherein the second surface spectral reflectance isshown as a function of wavelength of incident light. Even though mirror40 has no dark colored or black backing at all, it still is silvery incolor when viewed from the front side.

As indicated previously, thin film opacification means can be used forboth the first and second surface mirrors of both the achromatic andspectrally selective type. For example, an opacified achromatic firstsurface mirror can be constructed by using a thin film design comprisingan outer reflector layer of silicon elemental semiconductor, of opticalthickness 800 angstroms or thereabouts, which is opacified with anunderlying additional thin film means consisting of a thin film ofsilicon dioxide, of optical thickness 428 angstroms or thereabouts,followed by a thin film of silicon elemental semiconductor, of opticalthickness of at least 4000 angstroms or thereabouts and preferably of atleast 6800 angstroms or thereabouts, all deposited onto the frontsurface of the glass substrate. The optical design is thus air/silicon(800 angstroms optical thickness)/silicon dioxide (428 angstroms opticalthickness)/silicon (6800 angstroms optical thickness)/glass and issimilar to that in FIG. 9A. Further, for either first or second surfacemirrors, the additional thin film means are disposed to the rear of theelemental semiconductor reflector layer. Such opacified first surfacemirrors have commercial advantage in that the reflector layer andadditional thin film opacification means are deposited on the same glasssurface, thus facilitating coating deposition onto glass substratesbeing conveyed past a sputtering target, for example, and in that asingle type of sputtering target, for example, a silicon target in thedesigns above, can be used to deposit all the layers in the thin filmconstruction.

Within the context of this invention, full or high reflectivity is amirror reflectance that approaches as close as is practical the 90% orthereabouts luminous reflectance conventionally provided bysecond-surface silvered mirrors. Car drivers, particularly in the UnitedStates, have for decades opted to use exterior glare-reducing rearviewmirrors which use a thin film of chromium metal as their reflectorelement and thereby achieve a reflectance level (of 55±5% orthereabouts) which is moderate and offers a compromise between daytimevisibility and nighttime glare protection. However, truck drivers ingeneral, and some car drivers, particularly in Europe, have continueduse of silvered, high reflectance mirrors. They do so because theyparticularly value the extra rear vision performance of such highreflectance mirrors and are willing to suffer the excessive glarereflected off said high reflectance mirrors. Thus, some drivers desire ahigh reflectance mirror with at least 60% luminous reflectancedesirable, and greater than 70% preferred. To meet these driver'spreferences, another embodiment 46 of an opacified achromatic mirror wasmade as shown in FIG. 9A comprising a first surface mirror having afirst layer 47 of elemental silicon coated onto a glass substrate 12 toan optical thickness of about 6950 angstroms, followed by deposition ofa thin film 48 of silicon dioxide at an optical thickness of 1050angstroms, followed by deposition of a thin film 49 of elemental siliconto an optical thickness of about 1600 angstroms. Mirror 46 had aluminous reflectance of 69% before heating and bending and, as shown inFIG. 9B, a luminous reflectance of 74% after heating and bending. Thisconvex, curved mirror then offers those drivers who desire to use a highreflectance mirror the advantages of scratch and wear resistance offeredby the elemental semiconductor mirror.

A seventh embodiment 50 of the mirror incorporating an elementalsemiconductor layer is shown in FIG. 10. Mirror 50, where like numeralsindicate like parts, is a spectrally selective, glare-reducing mirror inwhich the spectrally selective color, namely, blue, being reflected isstrong and quite noticeable to the viewer. In mirror 50, substrate 12includes a first layer 52 of elemental silicon semiconductor preferablyvacuum sputter coated on first surface 14 to an optical thickness ofbetween about 345 and 2400 angstroms, and preferably between about 720and 920 angstroms corresponding to a coating thickness of between about180 and 230 angstroms at n=4. If elemental germanium is used, layer 52will have an optical thickness of at least about 275 angstroms. Appliedover the front surface of silicon layer 52 is a relatively thicker,interference layer 54 of a dielectric material, namely, silicon dioxide(also known as silica) having an optical thickness of at least about 500angstroms, and preferably of between about 1600 and 2800 angstroms, andmost preferably of about 2250 angstroms corresponding to a coatingthickness of about 1500 angstroms and an index of refraction of aboutn=1.5. Finally, a light absorbing coating 22 is applied as describedabove to rear surface 16 of the substrate.

Preferably, silicon dioxide coating 54 is applied in a vacuum sputtercoating chamber in a manner similar to that for silicon layer 20 asdescribed above for mirror 10. In addition, silicon dioxide layer 54 canbe applied in the same vacuum sputter coating chamber as silicon layer52 if there is sufficient separation of the coating compartments toeliminate any oxygen liberated or employed purposely for the silicondioxide coating process being involved in the relatively oxygen freeenvironment present in the coating chamber wherein silicon semiconductordeposition occurs. Preferably, silicon dioxide layer 54 is formed byradio frequency (RF) or direct current (DC) reactive sputtering using asilicon elemental semiconductor target within a sputtering chamberincluding an atmosphere of an inert gas mixture such as neon, argon orthe like and oxygen. Alternately, the atmosphere may be pure oxygen. Thesputtering in such chamber creates an oxidized layer of thesemiconductor target material. Alternately, silicon dioxide may be radiofrequency sputtered from a fused quartz or silicon dioxide (SiO₂) targetin an atmosphere of argon, neon, oxygen, or mixtures thereof or othergas used as a sputter discharge support gas. RF sputtering isparticularly useful if the silicon elemental semiconductor target is notvery electrically conductive or if silicon dioxide, which is anelectrical insulator, is used. If, however, the silicon elementalsemiconductor target is doped with phosphorous as mentioned above, thenthe conductivity may be sufficient for use of DC sputtering.

Examples of other thin film dielectric materials suitable for our mirrorapplications are fluorides, of which cryolite and magnesium fluoride areexemplary, other oxides such as silicon monoxide, cerium oxide, tantalumoxide, titanium dioxide, and aluminum oxide, sulfides like zinc sulfideand nitrides such as silicon nitride. Such dielectrics can be depositedby vacuum deposition or other techniques such as dip coating, to producethin film layers of refractive index between about 1.3 and 2.7.

As an alternative to the spectrally selective mirror 50 shown in FIG.10, a similar spectrally selective mirror 60 is shown in FIG. 11 wherelike parts are indicated by like numerals. Mirror 60 includes atransparent glass substrate 12 having a thin layer of elemental siliconsemiconductor 62 on front surface 14. Layer 62 is applied preferably byvacuum sputtering as described above or alternately by other vacuumdeposition methods. Next, a silicon dioxide layer 64 is dip-coated orspin coated on at least the coated surface of substrate 12. In somecases, similar thin film layer 66 may be simultaneously dip-coated onrear surface 16. Alternately, rear surface 16 may be covered to preventcoating when dipped. When substrate 12 is soda-lime glass, the silicondioxide or silica layers 64, 66 have a refractive index close to as thatof the glass. Hence, little or no reflection occurs at the silica/glassinterface so that dip-coated silicon dioxide layer 66 is coated on rearsurface 16 of silicon coated glass with no appreciable effect on theoptics. Yet, layer 64 which overcoats the silicon 62 on the front sideof the substrate, has an appreciable interference optical effectcreating the spectral selectivity desired for the mirror. After theabove dip coating of layers 66 and/or 64, a light absorbing layer 22 canbe applied to the rearmost surface over dip-coated silica layer 66 orrear surface 16 in the manner described above or, alternately, can beomitted if substrate 12 is an opaque glass or ceramic or othersubstrate.

The above dip coating method for applying layers 64, 66 of silicondioxide may be accomplished by the dip and fire technique whereinsubstrate 12 including coating 62 thereon is dipped into a solution ofan appropriate precursor of silicon dioxide dissolved in a suitablesolvent. For example, a solution formed by dissolvingtetraethylorthosilicate in alcohols can be used. Upon withdrawal fromthis solution, the solution evenly wets one or both surfaces, i.e., thecoated first surface and the uncoated second surface, of the substratedepending on whether the second surface is covered when dipped. Thecoating on the first and second surfaces is then fired in an oven atabout 450° C. for about 60 minutes, or thereabouts, to completehydrolysis and condensation and to densify the newly formed oxidecoating. If curved glass is desired, either the dipped coating or thedip/fire oxide coating can be bent in a conventional bending process.The thicknesses of the dip-coated silicon dioxide films can be adjustedby modifying the withdrawal rate from the oxide precursor solution. Thefaster the withdrawal rate, the thicker the film on the coated anduncoated substrate sides.

As an example of mirrors 50 and 60 shown in FIGS. 10 and 11, thespectral reflectance for a similar mirror in the visible region is shownin FIG. 12. A 680 angstrom optical thickness layer of elemental siliconsemiconductor, corresponding to a coating thickness of about 170angstroms at n=4 was deposited on the front surface of a glass substrateby DC sputtering from a sputtering target comprising a composition of 6%aluminum and 94% silicon using neon sputtering gas at a pressure of 3mTorr. Thereafter, a 2160 angstrom optical thickness layer of silicondioxide dielectric corresponding to a coating thickness of about 1440angstroms at n=1.5, was deposited as an interference coating by radiofrequency sputtering from a silicon dioxide sputtering target usingargon sputtering gas at a pressure of 3 mTorr at an RF power of 2 kW.This coated glass was then bent in a conventional glass bendingapparatus by heating to at least 450° C. for between a few minutes andan hour followed by pressing the heated, coated glass in a curved mold.At this point, the luminous transmittance was 38.9% and the luminousfirst surface reflectance was 42.7%. The rear or second surface was thencoated with a BLACK BASE COAT paint manufactured by Lilly IndustrialCoatings of Indianapolis, Ind. The resultant mirror had a luminousreflectance of 42.1% which is only slightly lower than before applyingthe paint and is suitable for use as an exterior automotive rearviewmirror world wide. The thicknesses of both the silicon and silicondioxide layers provide such reflectance and can be adjusted to changethe luminous reflectance. For example, if the optical thickness ofsilicon elemental semiconductor layer 52 or 62 is decreased to belowabout 680 angstroms but remains above 345 angstroms, corresponding to acoating thickness of 170 angstroms and 86 angstroms, respectively, atn=4, while the silicon dioxide layer remains at an optical thickness ofabout 2250 angstroms corresponding to a coating thickness of about 1500angstroms, the luminous reflectance will decrease below 40% as theelemental silicon layer gets thinner.

For comparison purposes, the luminous reflectance of a commerciallyavailable spectrally selective blue mirror marketed by DonnellyCorporation of Holland, Mich. under the name "BLUE MIRROR" using amultilayer coating having a nearly opaque metal therein is shown in FIG.13. The peak reflectance occurs in both mirrors between 400 and 500 nmin the blue range of the visible spectrum and falls off sharplythereafter in the yellow/red portions of the spectrum. As indicated, theresponse is sufficient for the mirror similar to that of mirrors 50 and60 to provide acceptable spectral response with an already commerciallyaccepted rearview mirror.

Another rearview mirror 70 incorporating the present invention andcomprising a first surface spectrally selective mirror is shown in FIG.14 where like numerals indicate like parts in mirrors 10, 25, 30, 35,40, 50 and 60. Mirror 70 includes a transparent or opaque substrate 12,preferably formed from soda-lime glass or ceramic, a thin layer ofelemental silicon semiconductor 72 on first or front surface 14, and asecond layer 74 of another dielectric material, namely, silicon nitridecoated on the front surface of silicon layer 72. A light absorbing layer22 of the type described above is applied by one of the above-describedmethods to rear surface 16 to prevent secondary reflections as mentionedabove. Silicon layer 72 preferably has an optical thickness of betweenabout 800 to 1200 (coating thickness of 200 to 300 angstroms at n=4)angstroms, while silicon nitride layer 74 has a preferred opticalthickness of 1600 to 2800 angstroms (coating thickness of 800 to 1300angstroms at n=2.0). Silicon nitride is a useful interference coating toprovide the spectral selectivity desired for the mirror since its indexof refraction (n) is about 2.0 to 2.2. As with layers 54, 64 in mirrors50, 60, it is desired that the interference coating have an opticalthickness equal to approximately one-quarter of the wavelength of theregion of the visible spectrum to be suppressed or spectrally selectedin the mirror.

The spectral reflectance of a mirror made in accordance with mirror 70is shown in FIG. 15 calculated for a soda-lime glass substrate coatedwith an optical thickness of about 1200 angstroms of silicon elementalsemiconductor as layer 72 (coating thickness of about 300 angstroms atn=4) followed by layer 74 of silicon nitride having an optical thicknessof about 2000 angstroms (coating thickness of about 1000 angstroms atn=2.0) and an n=2.0 to 2.2 over the silicon layer 72. The luminousreflectance is computed to be 40.3% and, thus, is useful as an exteriorrearview mirror in both the United States and Europe. Excellent spectralselectivity is obtained due to the high reflectance in the 400 to 500 nmregion or blue wavelength portion of the visible spectrum and the lowreflectance in the yellow/red portion of the spectrum above about 560 nmwavelengths.

In FIG. 16, another mirror 80 is shown comprising a spectrally selectiveblue mirror in which multiple thin coatings are formed on the secondsurface 16 of the transparent glass substrate 12. Mirror 80 includes athin layer 82 of silicon nitride coated to an optical thickness ofbetween about 1600 and 2800 angstroms corresponding to a coatingthickness between about 800 and 1400 angstroms when n=2.0, andpreferably an optical thickness of 2200 angstroms on rear surface 16 ofsubstrate 12, followed by layer 84 of elemental silicon semiconductorhaving an optical thickness between about 275 and 2400 angstromscorresponding to a coating thickness between about 68 and 600 angstromsat n=4, and preferably, an optical thickness of between about 500 and1200 angstroms. This is followed by another layer 86 of silicon nitridecoated to an optical thickness of about 2400 angstroms corresponding toa coating thickness of about 1200 angstroms at n=2.0. Again, as inprevious embodiments, a light absorbing layer 22 is coated over the rearsurface of the final silicon nitride layer 86 to reduce secondaryreflections and improve mirror performance. Light absorbing layer 22 canbe formed from any of the materials or coatings set forth above inconnection with mirror 10 or the other embodiments. As shown in FIG. 17,the spectral reflectance for mirror 80 with layers 82, 84, 86 havingoptical thicknesses of 2200, 1200 and 2400 angstroms, respectively,provides enhanced spectral reflectivity in the blue region of thevisible spectrum between 400 and 500 nm while the overall luminousreflectance of the entire mirror is about 38%. Reflectance in theyellow/red wavelength region above about 560 nm is significantly reducedshowing the usefulness of mirror 80 as a glare-reducing, spectrallyselective, blue mirror.

Although light absorbing coating 22 is shown as being only on the rearsurface of thin silicon nitride layer 86, it can be extended around theedges of layer 82, 84 and 86 to overlap the peripheral edge of substrate12 as shown in the dotted lines of FIG. 16. This helps to protect theedges of the thin layer coatings from elemental conditions such asmoisture, humidity, salt spray, car wash detergents, oxidation, abrasionand the like commonly encountered by vehicles. However, it has also beenfound that the elemental semiconductor layers of silicon or germanium inthe present invention have excellent inherent resistance to damage fromsuch environmental effects even without protective overcoatings of thistype. Indeed, a coating of silicon or germanium on the first surface ofa substrate as described above has significant resistance toenvironmental damage due to oxidation, salt, heat, humidity, abrasion,detergents and the like. As described below, the scratch resistance andhardness of single semiconductor layers especially of silicon can beimproved by heating the coated substrate such as in a lehr heatingconveyor line.

As shown in FIGS. 18 and 19, the present invention also encompassesspectrally selective rearview mirrors constructed of single layers of anelemental semiconductor preferably coated to an optical thickness ofwithin the range of between about 2400 and 10,000 angstroms depending onwhether silicon or germanium is used. For example, as shown in FIG. 18,mirror 90 includes a single layer 92 of elemental silicon semiconductorcoated to an optical thickness of about 4800 angstroms corresponding toa coating thickness of about 1200 angstroms at n=4 on the first surface14 of glass substrate 12. In addition, a light absorbing layer 22 suchas that described above for the other rearview mirror embodiments may becoated on rear surface 16 to reduce secondary reflections. Mirror 90,when constructed in this form, provides a spectral reflectance which isvisually blue and an overall luminous reflectance of about 31% as shownin FIG. 19. FIG. 19 also illustrates the enhanced reflectance in theblue region of the visible spectrum, namely, in the 400 to 500 nmregion.

Although luminous reflectance for mirror 90 shown in FIGS. 18 and 19 isbelow the 35% required for U.S. and European automotive rearview mirrorapplications, such mirror may be combined to produce an overall mirror100 (FIG. 20) including a high luminous reflectivity mirror panel 102positioned in a suitable frame or case 104 vertically above the reducedluminous reflectivity, glare-reducing mirror panel 90. Mirror 100 willprovide extra glare-reducing protection in high glare environments butpermit a driver to observe using the higher reflectance mirror 102 whendesired. Alternately, such a mirror can be used in countries outside theU.S. or Europe where regulations are different, or in applications wheresuch regulations do not apply.

It-is also possible to modify the visual color enhanced in thespectrally selective mirror such as that shown in FIGS. 18 and 19 byvarying the thickness of the single layer 92 of mirror 90. For example,by coating layer 92 to an optical thickness of about 9600 angstroms,corresponding to a coating thickness of about 2400 angstroms at n=4, amirror enhancing visibility in the blue-green region of the visiblespectrum from about 400 nm to 600 nm is produced.

An alternate embodiment 110 of a first surface, spectrally selective,blue mirror similar to mirror 50 but including additional thin filmopacification layers 116, 118 instead of a dark colored backing coatingon glass substrate 12 is illustrated in FIG. 21. In mirror 110, twoadditional thin film layers are added to the front surface of the coatedmirror. Thus, as shown in FIG. 21, a layer 112 of elemental silicon ofan optical thickness of at least 4000 angstroms, and preferably at anoptical thickness of between about 6800 and 10,000 angstroms,corresponding to a coating thickness of between about 1700 and 2500angstroms at n=4, is deposited as in previous embodiments onto glassalthough other substrate materials could be used. This is followed by asilicon dioxide layer 114 of an optical thickness of about 1050angstroms, corresponding to a coating thickness of about 700 angstromsat n=1.5, on the front surface of layer 112. In turn, layer 114 isfollowed by deposition of a second elemental silicon layer 116 of anoptical thickness of about 800 angstroms, corresponding to a coatingthickness of about 200 angstroms at n=4, followed by a second and finallayer 118 of silicon dioxide having an optical thickness of about 2250angstroms corresponding to a coating thickness of about 1500 angstromsat n=1.5.

The spectral reflectance of mirror 110 on glass with silicon layer 112at an optical thickness of about 6,800 angstroms and layers 114, 116,118 as described above is shown in FIG. 22 illustrating spectralselectivity in the blue regions of the visible spectrum and sufficientlyhigh reflectance for use as an automotive rearview mirror. The color ofmirror 110 in reflectance is blue and the luminous reflectance is 46.2%.In addition, mirror 110 is nearly opaque with a luminous transmittanceof 0.1%. Such a low transmittance obviates the need for a black backingof any sort, such that mirror 110 is compatible with existing firstsurface rearview mirrors where adhesives are already available forsecuring the mirror to a mirror casing and assembly which may eventuallybe installed on a vehicle.

In general, opacification or near opacification using additional thinfilm means may require adjustment with respect to other layers of theconstruction, particularly in second surface constructions. However, byusing opacification layers of elemental semiconductors and dielectricmaterials instead of other backing materials, the major advantage isrealized of allowing heating and bending after coating on flatsubstrates.

As will now be apparent, in all embodiments of the mirrors herein,silicon nitride or other heatable and bendable dielectric materialswhich do not degrade upon heating may be substituted for silicondioxide. Moreover, variations of the various optical and coatingthicknesses for each of the mirrors can be used while maintaining goodresults.

As mentioned above, the application of heat to the single elementalsemiconductor layers when coated on glass significantly improves theenvironmental and abrasion resistance for mirrors incorporating such alayer. For example, if first surface mirrors 10, 90 or 110 are desiredto have improved scratch and abrasion resistance, it is beneficial toheat the coated substrates to temperatures of at least about 200° C. orso for a short period of time. Heating to such temperature isaccomplished satisfactorily by, for example, increasing the temperatureto 450° C. in an oven in less than one hour followed by decreasing thattemperature in less than two hours to ambient/room temperatures whileproviding a relatively short soak time at the 450° C. level, i.e.,between about 0 and 30 minutes. Lower temperatures could be used butrequire longer heating periods for equivalent results. After suchheating, the elemental semiconductor coating on the substrate isenvironmentally resilient, hard and scratch resistant and, therefore,highly suitable for automotive rearview mirror use. Such heating can,for example, be conducted through a lehr heating conveyor line as isconventionally known.

Alternately, the improved hardness, resiliency and scratch resistance ofthe elemental semiconductor coating described above can be obtained byheating a sheet of glass to a temperature of about 200° C. prior tocoating the glass with the elemental semiconductor. One surface of theheated glass sheet may then be coated with a thin layer of an elementalsemiconductor such as silicon or germanium to a desired opticalthickness of at least about 800 angstroms which will also cause thesemiconductor coating to be environmentally hard, resilient and scratchresistant after the coated heated substrate cools.

Exemplary of the resiliency and scratch resistance of such coatings is acomparison of two otherwise identical elemental silicon thin filmscoated onto single strength soda-lime glass where one was retained asdeposited and the other was conveyed through a lehr furnace havingheating stages set for 550° C., 540° C. and 520° C. at a conveyor rateof 61 inches per minute. Both resultant coated glass substrates wereabraded with an eraser stroke tester well known in the art. The samplewhich was heated in the Lehr furnace process showed about the same levelof film damage at 3000 strokes of the tester as the unheated sample didat about 1000 strokes. Thus, although heating of the elemental siliconlayers need not be done, when heating is employed, either with orwithout subsequent bending, the resistance to environmental damage issignificantly improved making the improved coatings useful for rearviewmirrors where high quality and durability standards are common.

Alternately, the production economics of producing the mirrors of thepresent invention are increased over prior known mirror productionmethods due to the ability to heat and bend the elemental semiconductorcoating on glass without degrading the reflectivity thereof. Forexample, when the silicon film of one or more of the above mirrorembodiments is coated onto flat glass, the glass may then be heated andbent in a conventional bending process without significant degradationof the silicon film, i.e., without crazing, cracking or hazing, orreduction in reflectance. Such bending is conventionally done by heatingthe glass up to a temperature of at least 450° C. or thereabouts,following by conforming the glass to a metallic or other mold all as isknown to those in the art of glass bending. Alternately, shapes can becut from large, flat coated glass lites, followed by use of the flatshapes as individual pieces or heating and bending of the individualpieces to form, for example, convex exterior rearview mirrors forautomobiles. Alternately, bending can be done on the large, coated glasslites followed by cutting of the mirror shapes. The latter method isespecially economical by allowing the vacuum sputter coating of siliconlayers on large glass lites followed by heating to improve theresilience and scratch resistance of the layers after which the largeglass lite can be either bent or cooled to allow cutting into individualmirror shapes. If the glass lite is bent subsequent to heating, thelarge bent glass lite can then be cut into individual shapes for use asdesired. This method also allows the mass coating of numbers of largeglass lites for retention in inventory until needed to produceindividual flat mirror shapes or bent mirror shapes in one of the twoprocesses described above.

It is also possible without degrading the reflective character of themirror to produce curved mirrors by heating and bending after coating ofthe dielectric interference layers on top of or in combination with theelemental semiconductor layers of silicon or germanium again withoutcracking, hazing or crazing during such heating and bending. Inaddition, through the use of high temperature paint such as thosementioned above, the final light absorbing coating used to preventsecondary reflections may also be applied to the glass lites prior toheating and bending again without degradation of the coating during suchprocessing. It has been found that during such heating and bending, theelemental semiconductor coating does not become converted to its oxidesuch as silicon dioxide where the refractive index would dropsignificantly as would the mirror reflectance. Rather, high temperatureheating followed by bending of such substrates with combined coatingsallows the refractive index to remain high in the elementalsemiconductor layer such that mirrors of excellent reflective qualityare produced all without cracking, crazing or hazing during heating andbending.

In addition, the above method permits in-line, sequential processingwherein sequential processing units are aligned to maximizemanufacturing efficiency. For example, large glass sheets can be loadedonto a conveyor and washed in a glass cleaning unit which utilizesdetergent-assisted face cleaning, clean deionized water rinsing anddrying using air knives. This cleaned glass can be loaded into anin-line sputter coater where a thin elemental semiconductor coating canbe applied to glass in one of several parallel or integratedmanufacturing lines, i.e., one for flat mirror production, another forcurved, coated glass lite production, one for the cutting of shapesfollowed by bending of individual shapes, and yet another for applyingyet another coating to the glass that is on top of the already coatedsilicon layer such as an interference coating of dielectric material,namely, silicon dioxide or silicon nitride, as described above. Uponexiting the sputter coating chamber, the now elemental semiconductorcoated large glass sheets are, alternately, cut into mini-lites or intoshapes, are heated and bent before subsequent cutting, are cut intomini-lites before subsequent heating and bending, are subsequentlysputter coated with one or more additional interference layers, or arecoated with high temperature resistance or other protective, lightabsorbing paint. Depending on the type of paint, the shapes may thenpass through a final baking station after which they are packed forcustomer use. The interference coating may also be applied to theopposite side of the substrate during such processing.

Finally, it is noted that for either large or individual sized glasslites coated with elemental semiconductor layers, such mirrors can beheated and bent in the manner described and stock-piled in inventoryuntil needed. Thereafter, the additional dielectric interference layerscan be sputter coated over the previously applied semiconductor layersto produce the spectrally selective mirrors described above.

As yet another alternative, it is possible to apply dielectricinterference layers such as silicon nitride layer 82 in mirror 80 to aglass substrate or lite followed by heating and bending of that coatedsubstrate with dielectric layer 82 thereon without crazing, cracking orhazing or other degradation of the dielectric layer. Thereafter, a layerof elemental silicon or germanium semiconductor may be sputter coatedover top the previously applied dielectric layer as in layer 84 followedby the application of yet another dielectric layer such as layer 86 ofsilicon nitride where the first layer 62 is also of silicon nitride.Likewise, the additional, thin film opacification layers such as thoseat 44, 46 in mirror 40 or at 116, 118 in mirror 110 can also be appliedbefore heating and bending without degradation by such furtherprocessing. Accordingly, the economic method of coating, heating andbending without degradation of the coating allows many and variedmanufacturing processes for maximum efficiency.

While several forms of the invention have been shown and described,other forms will now be apparent to those skilled in the art. Therefore,it will be understood that the embodiments shown in the drawings anddescribed above are merely for illustrative purposes, and are notintended to limit the scope of the invention which is defined by theclaims which follow.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method formanufacturing a glare-reducing mirror for vehicles comprising the stepsof:1) providing a sheet of flat glass having front and rear surfaces; 2)coating one surface of said sheet with a thin layer of an elementalsemiconductor having an index of refraction of at least 3 to a desiredoptical thickness of at least about 275 angstroms; 3) heating saidcoated glass to a temperature sufficient to allow bending of said coatedglass; and 4) bending said heated, coated glass to a desired curvature;said coating step including coating said one sheet surface with anelemental semiconductor selected from the group including silicon andgermanium; and coating said layer of elemental semiconductor with a thinlayer of dielectric material of optical thickness within the range ofbetween about 1600 and 2800 angstroms prior to heating and bending saidcoated glass.
 2. The method of claim 1 wherein prior to said heating andbending steps, said coating with said dielectric material includesdipping said coated sheet in a precursor of said dielectric materialdissolved in a solvent such that at least the front surface of saidcoated sheet is wetted with the solution; and firing said wetted coatedsheet to a temperature sufficient to reduce said solution on said frontsurface to an oxide forming said dielectric material.
 3. The method ofclaim 2 including coating the other surface of said sheet with a layerof light absorbing material.
 4. The method of claim 1 including coatingthe rearmost surface of said coated sheet prior to said heating andbending steps with a layer of light absorbing material which is heatresistant.
 5. A method for manufacturing a glare-reducing mirror forvehicles comprising the steps of:1) providing a sheet of flat glasshaving front and rear surfaces; 2) coating one surface of said sheetwith a thin layer of an elemental semiconductor having an index ofrefraction of at least 3 to a desired optical thickness of at leastabout 275 angstroms; 3) heating said coated glass to a temperaturesufficient to allow bending of said coated glass; and 4) bending saidheated, coated glass to a desired curvature; said coating step includingcoating said one sheet surface with an elemental semiconductor selectedfrom the group including silicon and germanium; and coating said layerof elemental semiconductor with a thin layer of a dielectric material ofoptical thickness of about 1050 angstroms prior to heating and bendingsaid coated glass; and coating said thin layer of dielectric materialwith an additional thin layer of elemental semiconductor and anadditional thin layer of dielectric material prior to heating andbending said coated glass, said additional thin dielectric materiallayer being the closest of said additional layers to the direction fromwhich said mirror is viewed.
 6. The method of claim 5 wherein saidadditional thin elemental semiconductor layer is silicon having anoptical thickness of about 800 angstroms, and said additional thindielectric layer is silicon dioxide having an optical thickness of about2250 angstroms.
 7. A method for manufacturing a glare-reducing mirrorfor vehicles comprising the steps of:1) providing a sheet of flat glasshaving front and rear surfaces; 2) coating one surface of said sheetwith a thin layer of an elemental semiconductor having an index ofrefraction of at least 3 to a desired optical thickness of at leastabout 275 angstroms; 3) heating said coated glass to a temperaturesufficient to allow bending of said coated glass; and 4) bending saidheated, coated glass to a desired curvature; said coating step includingcoating said rear surface of said sheet with a thin layer of dielectricmaterial of a thickness within the range of between about 1600 and 2800angstroms followed by coating the rear surface of said thin dielectriclayer with said elemental semiconductor layer prior to heating andbending said coated glass; said elemental semiconductor being selectedfrom the group including silicon and germanium, and coating the rearmostsurface of said coated sheet prior to said heating and bending stepswith a layer of light absorbing material which is heat resistant; saidmethod providing a mirror with a luminous light reflectance of at leastabout 30% of the light incident thereon from the direction of said frontsurface of said sheet; said heating and bending of said coated glass toa desired curvature being achieved Without substantial cracking, crazingand hazing of said thin elemental semiconductor layer.